UNIVERSITY  OF  CALIFORNIA 

COLLEGE    OF    AGRICULTURE 

BERKELEY 


Agricultural  Experiment  Station 

BENJ.    IDE    WHEELER,    President 
THOMAS    FORSYTH    HUNT,  Dean  and  Director 
H.  E.  VAN    NORMAN,    Vioe-Director    and    Dean 
University  Farm  School 


CIRCULAR  No.  125 

January,  1915 


APHIDS  ON  GRAIN  AND  CANTALOUPES* 


BY 
C.  W.  WOODWOETH 


The  two  most  serious  insect  pests  in  the  Imperial  Valley  are  the 
grain  aphis  and  the  cantaloupe  aphis.  Both  of  these  insects  are  well 
known  in  other  regions  and  both  have  a  long  list  of  other  plant  foods. 

The  life  histories  run  as  follows :  The  young  are  produced  alive, 
sometimes  as  many  as  eight  a  day  and  they  become  full  grown  and 


Anal  plates  and  honey  tubes  of  the  grain  and  cantaloupe  aphids,  the  latter 

on  the  right. 


begin  to  reproduce  within  two  weeks,  often  considerably  less.  Most 
of  the  individuals  are  wingless,  but  occasionally  a  winged  insect 
appears.  The  rate  of  reproduction  given  by  the  U.  S.  Department  of 
Agriculture  (Bulletin  112)  shows  that  if  favorable  conditions  con- 
tinued and  all  the  descendants  could  live,  the  progeny  of  a  single 
individual  grain  aphis  in  six  months  would  make  a  mass  sufficient 
to  cover  the  whole  State  of  California  a  foot  thick. 


*  The  two  aphids  are  Aphis  avenae  and  Aphis  gossypii  Glov.  and  are  generally 
called  the  oat  aphis  and  the  cotton  aphis,  as  indicated  by  their  Latin  names. 
The  common  wingless  forms  are  readily  distinguishable  by  the  hairing  of  the 
anal  plate  and  the  shape  of  the  honey  tubes  as  shown  in  the  figure. 


Such  figures  aid  us  in  understanding  how,  when  favorable  condi- 
tions occur,  the  aphids  can  increase  to  countless  millions  and  destroy 
a  crop. 

Fortunately  the  aphids  develop  very  slowly  in  the  colder  period 
in  the  spring  and  very  frequently  the  weather  becomes  too  severe 
long  before  harvest,  leaving  but  a  very  short  period  during  which 
rapid  development  is  possible.  Most  of  the  time  these  insects  lead 
precarious  existences  but  now  and  then,  and  with  unpleasant  regularity 
in  the  Imperial  Valley,  the  weather  conditions  and  food  supply  permit 
the  reproduction  of  the  insect  to  approach  its  maximum  rate. 

The  cantaloupe  aphis  is  almost  annihilated  when  the  vines  die  in 
the  early  summer,  but  a  few  survive  on  alfalfa  or  various  weeds, 
particularly  pepper  grass,  and  the  orange  is  fairly  favorable  while 
the  young  shoots  are  growing. 

The  grain  aphis,  during  the  unfavorable  periods,  often  penetrates 
the  ground  for  shelter  and  finds  food  on  the  roots  of  plants,  chiefly 
grasses,  and  in  fall  or  spring  it  often  goes  to  the  leaves  of  the  apple 
and  passes  one  or  more  generations  there.  Sexual  forms  occur  in  the 
autumn  in  all  northern  sections  and  eggs  are  produced  to  carry  the 
insect  over  the  winter.  This  occurs  only  in  the  generation  produced 
on  the  apple.  We  have  not  noticed  this  wintering  egg  form  in  the 
Imperial  Valley  and  the  cantaloupe  aphis  has  never  been  observed 
producing  eggs. 

Parasites  and  predaceous  insects  have  often  been  given  the  credit 
of  controlling  aphids,  but  a  knowledge  of  the  enormous  rate  of  repro- 
duction in  these  insects  will  show  that  when  conditions  are  right  for 
increase  those  insects  that  prey  upon  them  can  make  very  little 
impression.  When  an  aphis  is  on  the  down  grade,  however,  the 
attacks  of  other  insects  seem  very  efficient  since  the  aphids  cannot  in 
any  manner  defend  themselves  or  avoid  wholesale  destruction. 

The  experiment  of  importing  ladybirds  into  the  Imperial  Valley 
has  been  carried  out  on  a  larger  scale  than  anywhere  in  the  world, 
but  the  growers  in  that  valley  know  that  the  aphids  still  collect  toll 
in  no  diminishing  ratio. 

Sprays  are  very  effectively  used  against  aphids.  They  are  perhaps 
the  easiest  of  all  insects  to  kill  in  this  way.  AVhile  these  aphids  are 
quite  as  defenseless  against  sprays  as  against  ladybirds  they  cannot 
usually  be  controlled  by  a  single  treatment  because  of  their  great 
reproductive  powers.  The  descendants  of  a  single  individual  that 
escaped  contact  with  the  spray  material  would  be  able  to  seriously 
reinfest  a  plant  in  an  incredibly  brief  space  of  time,  thus  requiring 


repeated  treatments  as  long  as  the  conditions  for  rapid  reproduction 
persist. 

Whether  sprayed  or  not  plants  often  suddenly  become  free  from 
aphids  when  a  change  of  weather  produces  a  less  favorable  condition. 
The  frequent  recurrence  of  such  weather  changes  is  accountable  for 
most  of  the  mistakes  made  by  careless  observers  who  do  not  notice  that 
the  conditions  are  general  and  not  limited  to  particular  fields  where 
ladybirds  or  other  inefficient  means  of  control  have  been  experimented 
with. 

In  most  districts  the  aphids  on  any  particular  crop  give  trouble 
only  during  occasional  seasons  and  the  remainder  of  the  time  are 
negligible  factors. 

Gardeners  do  not  consider  aphids  difficult  insects  to  combat  because 
under  the  worst  conditions  the  necessary  spraying  is  neither  difficult 
nor  expensive,  even  though  requiring  many  repetitions  of  the 
treatment. 

The  formula  most  frequently  used  consists  of  one  part  of  40% 
nicotine  and  one  part  of  soap  to  900  parts  of  water.  This  is  cheap 
and  effective. 

In  field  crops  the  cost  of  treatment,  though  one  of  the  cheapest 
methods  known  for  killing  insects,  is  generally  considered  prohibitive. 
It  would  seem  that  the  value  per  acre  of  the  cantaloupe  crop  should 
justify  the  treatment  for  the  aphis  but  here  we  have  a  peculiar  con- 
dition prevailing. 

In  the  cantaloupe  fields  in  the  Imperial  Valley  the  aphids  do  not, 
as  a  rule,  become  generally  abundant  enough  to  seriously  affect  pro- 
duction till  after  the  more  profitable  portion  of  the  crop  has  been 
harvested.  The  grower  will  have  to  decide  whether  the  continuation 
of  the  season,  by  spraying  the  plants  and  thus  keeping  them  pro- 
ductive, is  profitable  with  the  decreasing  market  value  of  the  melons 
which  is  always  experienced  at  that  time  of  the  year.  The  question 
before  the  grower  is  not  primarily  entomological,  having  to  do  with 
the  method  of  killing  the  aphid,  but  almost  entirely  a  question  of  farm 
management,  whether  it  pays  to  invest  the  additional  cost  of  produc- 
tion to  increase  the  volume  of  the  least  profitable  shipments. 

Those  who  have  tried  spraying  are  far  from  convinced  that  they 
have  sufficient  returns  for  the  money  expended  and  we  would  recom- 
mend that  any  further  experimenting  be  done  on  a  small  scale,  rather 
than  by  the  treatment  of  whole  fields. 

There  is,  however,  one  practice  which  has  developed  in  the  Imperial 
Valley  which  seems  to  be  justified  by  the  experience  of  the  growers. 
When  the  vines  first  become  infested  in  the  spring  it  is  usual  to  find 


the  insect  limited  to  a  very  small  number  of  vines  upon  which  they 
become  exceedingly  abundant  before  developing  wings  and  spreading 
generally  over  the  field.  When  this  is  the  case  the  practice  of  many 
growers  is  to  sprinkle  gasolene  on  each  of  these  early  infested  vines 
and  set  fire  to  it,  thus  killing  all  the  aphids  as  well  as  the  plant.  This 
practice  does  not  prevent  the  final  general  infestation  of  the  field,  but 
delays  the  time  of  infestation  very  appreciably.  Where  this  aphis 
gives  trouble  in  other  parts  of  the  State  it  would  be  well  to  adopt  this 
method  of  controlling  the  insect,  provided  the  winged  spring  migrants 
are  as  few  as  is  the  rule  in  the  Imperial  Valley,  with  the  result  that 
they  are  able  to  infest  only  a  very  small  proportion  of  the  vines.  If 
the  early  infestation  is  general  this  method  is  of  no  value. 

Regarding  the  grain  aphis  it  is  clear  that  spraying  does  not  pay 
and  it  will  have  to  be  considered  as  one  of  the  risks  in  growing  the 
crop,  belonging  in  the  same  class  as  the  direct  effects  of  damaging 
weather  conditions.  It  has  never  been  considered  profitable  in  any 
country  to  apply  treatment  for  these  insects  on  grain  and  the  con- 
ditions in  the  Imperial  Valley  do  not  offer  any  reason  for  expecting 
it  to  be  practical  there. 


